Reduced fluidization of solid particles in radial flow fluid/solid contacting

ABSTRACT

Apparatuses and methods are disclosed for contacting radially flowing fluids with solid particles (e.g., catalyst) with reduced tendency for fluidization of the particles, and especially a sealing portion of the particles at the top of a particle retention zone disposed between screens at upstream and downstream positions relative to radial fluid flow. Fluidization is reduced or eliminated by offsetting openings of the screens in the axial direction, such that upstream openings in the upstream screen are above highest downstream openings in a downstream stream. The offset in openings imparts a downward flow component to radially flowing fluid, thereby reducing solid particle fluidization without the need to induce a specific pressure drop profile along the entire axial direction of the screens.

CROSS REFERENCE TO RELATED APPLICATION

This application is Continuation of prior copending U.S. applicationSer. No. 12/824,431 filed on Jun. 28, 2010, the contents of which arehereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to processes for horizontal (e.g., radial)flow fluid/solid contacting processes in which fluidization of solidparticles (e.g., catalyst) is reduced. Openings in upstream anddownstream screens for retaining the solid particulate are offset withrespect to their vertical (e.g., axial) positions.

DESCRIPTION OF RELATED ART

A wide variety of industrial applications involve horizontal or radialflow apparatuses for contacting a fluid with a solid particulate.Representative processes include those used in the refining andpetrochemical industries for hydrocarbon conversion, adsorption, andexhaust gas treatment. In reacting a hydrocarbon stream in a radial flowreactor, for example, the feed to be converted is normally at leastpartially vaporized when it is passed into a solid particle catalyst bedto bring about the desired reaction. Over time, the catalyst graduallyloses its activity, or becomes spent, due to the formation of cokedeposits on the catalyst surface resulting from non-selective reactionsand contaminants in the feed.

Moving bed reactor systems have therefore been developed forcontinuously or semi-continuously (intermittently) withdrawing the spentcatalyst from the catalyst retention or contacting zone within thereactor and replacing it with fresh catalyst to maintain a requireddegree of overall catalyst activity. Typical examples are described inU.S. Pat. No. 3,647,680, U.S. Pat. No. 3,692,496, and U.S. Pat. No.3,706,536. In addition, U.S. Pat. No. 3,978,150 describes a process inwhich particles of catalyst for the dehydrogenation of paraffins aremoved continuously as a vertical column under gravity flow through oneor more reactors having a horizontal flow of reactants. Anotherhydrocarbon conversion process using a radial flow reactor to contact anat least partially vaporized hydrocarbon reactant stream with a bed ofsolid catalyst particles is the reforming of naphtha boilinghydrocarbons to produce high octane gasoline. The process typically usesone or more reaction zones with catalyst particles entering the top of afirst reactor, moving downwardly as a compact column under gravity flow,and being transported out of the first reactor. In many cases, a secondreactor is located either underneath or next to the first reactor, suchthat catalyst particles move through the second reactor by gravity inthe same manner. The catalyst particles may pass through additionalreaction zones, normally serially, before being transported to a vesselfor regeneration of the catalyst particles by the combustion of coke andother hydrocarbonaceous by-products that have accumulated on thecatalyst particle surfaces during reaction.

The reactants in radial flow hydrocarbon conversion processes passthrough each reaction zone, containing catalyst, in a substantiallyhorizontal direction in the case of a vertically oriented cylindricalreactor. Often, the catalyst in such a reactor is retained in theannular zone between inner and outer screens in the forms of concentriccylinders. The screens form a flow path for the catalyst particlesmoving gradually downward via gravity, until they become spent and mustbe removed for regeneration. The screens also provide a way todistribute gas or liquid feeds to the catalyst bed and collect productsat a common effluent or reaction product collection zone. In the case ofradial fluid flow toward the center of the reactor, for example, thiscollection zone may be a central, cylindrical space within thedownstream (in this case inner) screen. Regardless of whether the radialfluid flow is toward or away from the center, the passage of vapor isradially through an upstream (outer or inner) screen, the bed ofcatalyst particles, and through a downstream (inner or outer) screen.Some current state-of-the-art designs for moving catalyst beds, forexample, utilize screen made of profile wire (or V-shaped wire) having atriangular cross section to contain the catalyst.

Experience has shown, however, that the top of the bed of the solidparticles is susceptible to fluidization by the horizontally movingfluid that passes through the upstream screen, the solid particles, andthe downstream screen. This fluidization is a major concern, as itaccelerates particle attrition/fines formation and reduces theefficiency of contacting between the phases (e.g., due to bypassing) andtherefore the overall process (e.g., hydrocarbon conversion)performance. An excessive production of fines material undesirablyincreases the overall pressure drop of the fluid/solid contactingprocess and interferes with the withdrawal of the solid particles inmoving bed systems described above. Continued increase of the pressuredrop can necessitate suspending the process for maintenance/servicingand the associated, high costs of lost production time.

Current attempts to prevent fluidization at the top of the bed of solidparticles have focused on the use of a sealing portion of the particles(e.g., so-called “seal catalyst”) in the annular region between theupstream and downstream screens, but above the uppermost or highestopenings of these screens. The sealing portion is therefore a top,quiescent section of the annular particle retention zone, positionedabove the immediately lower section that is vertically or axiallyaligned with openings of the screens and therefore subjected to the flowof fluid. In some cases, the use of a sealing portion of the particlesmay be combined with passing a sealing portion of the fluid (e.g.,so-called “seal gas”) vertically or axially downward through theparticle bed, rather than radially. The flow of the sealing portion ofthe fluid further reduces the tendency to form localized upward flowcurrents in the major portion of the fluid flowing generally in theradial direction. A localized upward flow of fluid is normally observednear the upstream screen that is first contacted by the fluid to causelocal fluidization of the solid particles at the top of the bed.

The combination of sealing portions of both the solid particles andfluid can provide some benefit in terms of reducing the fluidizationproblems described above. Nevertheless, by adopting these strategies,the reduction in fluidization is obtained at the expense of greaterutilization of both fluid and particulate to provide these additionalsealing portions. Moreover, a satisfactory decrease in fluidization ofthe solid particles is not always realized with conventional methods. Aneed therefore exists, in the art of radial flow processes forcontacting a fluid with a solid particulate, to minimize or eliminatefluidization of the particulate, in addition to the excess volumes orvolumetric flows of sealing portions of the solid and fluid.

SUMMARY OF THE INVENTION

The present invention is directed to radial flow processes forcontacting fluids with solid particles that effectively controlfluidization of the particulate and its adverse consequences asdiscussed above. More particularly, the invention is associated with thediscovery that offsetting the elevations of openings in upstream anddownstream screens, used to contain the solid particles, effectivelyreduces localized upward gas flow near the top of the screens thatcontributes to fluidization. This offset screen configuration mayadvantageously be combined with the use of sealing portions of bothsolid particles and fluid, as discussed above, which further act tohinder undesired fluidization. Therefore, when used according to thepresent invention, a sealing portion of the solid particles may be atopmost portion above the highest openings of the upstream anddownstream screens. In this case, a line segment connecting highestopenings of the upstream and downstream screens can define a diagonalbase of the sealing portion. A top free surface, which may be curved(e.g., due to the addition of free falling particles from above), or mayotherwise be a relatively constant level of solid particles, correspondsto the height of the annular particle retention zone between the screensand above highest openings of both screens. This top free surface candefine the top of the sealing portion of solid particles, as well as thetop of the entire bed of solid particles. A sealing portion of the fluidmay be a portion of the total fluid (e.g., a hydrocarbon feed oroxygen-containing catalyst regenerant) entering a reactor or other typeof vessel to contact the solid particles (e.g., a hydrocarbon conversioncatalyst), whereby the sealing portion is directed axially downwardthrough the particle retention zone between the screens (e.g., anannular catalyst bed), and is therefore usually directed toward the topof the sealing portion of the particles, as defined above.

The present invention may be advantageously practiced without the needto induce specific pressure drop profiles at various screen elevations,according to conventional teachings for diverting radial fluid flowbetween the screens in the downward direction. See, for example, U.S.Pat. No. 4,959,198. Therefore, the pressure drop of fluid across one orboth of the upstream and downstream screens may be minimized, forexample, to a value that is generally less than about 20%, typicallyless than about 10%, and often less than about 5%, of the pressure dropof radially flowing fluid across the bed of solid particles. Accordingto preferred embodiments, the pressure drop across the upstream screenmay be characterized in this manner. In yet other embodiments, thepressure drop of fluid across one or both of the upstream and downstreamscreens, and preferably the upstream screen, is not graduated as afunction of the vertical or axial screen height, or at least notgraduated over a major portion of the height.

Accordingly, embodiments of the invention are directed to processes forcontacting a solid with a fluid. Representative processes comprisepassing at least a portion of the fluid substantially horizontally(e.g., substantially radially) through a bed of solid particles disposedbetween a vertically oriented upstream screen and a vertically orienteddownstream screen. The upstream screen has upstream openings above thehighest downstream openings of the downstream screen. Also, the pressuredrop of the fluid across each of these screens is as discussed above.

Particular embodiments are directed to processes for radial flowcontacting as described above, in which fluid is passed radially outwardfrom a central fluid distribution zone, internal to the upstream screen,to a peripheral fluid collection zone external to the downstream screen.While radial outflow is a preferred mode of operation, fluid may also bepassed radially inward from a peripheral fluid distribution zone,external to the upstream screen, to a central collection zone, internalto the downstream screen. In either case, the peripheral and centralzones within a vertically oriented vessel are annular and cylindrical,respectively.

Other particular embodiments are directed to hydrocarbon conversionprocesses utilizing radial flow contacting as described above, andrepresentative processes include the catalytic dehydrogenation ofparaffinic hydrocarbons (e.g., propane, butane, and/or isobutane) havingfrom 3 to 22 carbon atoms, as well as the catalytic reforming of naphthapetroleum fractions obtained from the fractionation of crude oil.

More specific embodiments of the invention are directed to processes forconverting a hydrocarbon feed that is at least partially in the gasphase. The processes comprise passing a major portion of the hydrocarbonfeed (e.g., to a central or peripheral fluid distribution zone asdescribed above) substantially radially through solid, hydrocarbonconversion catalyst that is retained in an annular catalyst bed within avertically oriented vessel. The catalyst is more particularly retainedbetween concentric, cylindrical upstream and downstream screens. Theprocesses further comprise passing a minor, sealing portion of thehydrocarbon feed axially downward from the top of a sealing portion ofthe catalyst and through the annular catalyst bed. The processesadditionally comprise withdrawing a hydrocarbon product (e.g., from acentral or peripheral collection zone as described above) that resultsfrom contact between the hydrocarbon feed (both the major portion andthe sealing portion) and the hydrocarbon conversion catalyst. Theprocesses also optionally comprise adding regenerated hydrocarbonconversion catalyst to the sealing portion of the catalyst andwithdrawing spent hydrocarbon conversion catalyst from the bottom of theannular catalyst bed, in order to provide an overall axial, downwardflow of the catalyst through the annular catalyst bed or catalystretention zone. The addition and/or withdrawal of catalyst may becontinuous or intermittent. An offset of the openings in the upstreamand downstream screens is used, as described above, in order to reduceor eliminate fluidization of the sealing portion of the catalyst. Also,the pressure drop of the hydrocarbon feed across each of the upstreamand downstream screens is as described above.

In any of the embodiments described above, the upstream and downstreamscreens preferably have flow channels formed from one or more elongatedparticle retention elements (e.g., profile wire).

These and other embodiments and aspects relating to the presentinvention are apparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side, cut-out view of representative, vertically orientedupstream and downstream screens that retain solid particles, including asealing portion above screen openings that are offset.

FIG. 2 is a more detailed, front view of a representative, cylindricalprofile wire screen, in which flow channels are formed between adjacent,spaced apart profile wires.

FIG. 3 is a side, cut-out view of a radial flow reactor with inwardradial flow and having both upstream and downstream screens disposedtherein and an annular catalyst bed retained within the annular spacebetween the concentric, cylindrical screens.

The features referred to in FIGS. 1-3 are not necessarily drawn to scaleand should be understood to present an illustration of the inventionand/or principles involved. Some features depicted have been enlarged ordistorted relative to others, in order to facilitate explanation andunderstanding. Upstream and downstream screens having highest openingsthat are offset, as well as radial flow fluid/solid contactingapparatuses and processes utilizing these offset screens, as disclosedherein, will have configurations, components, and operating parametersdetermined, in part, by the intended application and also theenvironment in which they are used.

DETAILED DESCRIPTION

Aspects of the invention relate to the configuration of solid particleretention devices, for example screens, for use in apparatuses for theradial flow contacting of fluids (e.g., gases, liquids, or mixed phasefluids containing both gas and liquid fractions) with solids that aretypically in particle form (e.g., spheres, pellets, granules, etc.). Themaximum dimension (e.g., diameter of a sphere or length of a pellet),for an average solid particle, is typically in the range from about 0.5mm (0.02 inches) to about 15 mm (0.59 inches), and often from about 1 mm(0.04 inches) to about 10 mm (0.39 inches). An exemplary solid particleis a catalyst used to promote a desired hydrocarbon conversion reactionand normally containing a catalytically active metal or combination ofmetals dispersed on a solid, microporous carrier.

Catalysts and other solid particles are retained in screens when thesmallest widths of the openings (e.g., flow channels), for passage offluid in the substantially horizontal (e.g., radial) direction, are lessthan the smallest dimension (e.g., diameter of a sphere or diameter ofthe base of a pellet), for an average particle. Typical smallest (orminimum) openings (or flow channel widths), for example, formed as gapsbetween adjacent, spaced apart profile wires or windings of profilewires) are in the range from about 0.3 mm (0.01 inches) to about 5 mm(0.20 inches), and often from about 0.5 mm (0.02 inches) to about 3 mm(0.12 inches). A representative apparatus containing both upstream anddownstream screens (e.g., comprising concentric cylinders having anannular catalyst bed or catalyst retention zone between the screens) istherefore a radial flow reactor that may be used in a number of chemicalreactions including hydrocarbon conversion reactions such as catalyticdehydrogenation and catalytic reforming.

FIG. 1 illustrates the use of a vertically oriented upstream screen 10 athat, in conjunction with vertically oriented downstream screen 10 b,retains a bed of solid particles, in a solid particle retention zone 14,between these screens 10 a, 10 b. As used herein, “retain” or“retention,” in describing the condition of the solid particles, refersto its confinement with respect to at least one dimension or direction(e.g., the horizontal dimension or radial direction) and does notpreclude its overall movement, for example in the vertical dimension oraxial direction. In fact, contemplated applications of the offsetscreens include their use in radial flow reactors in which the solidparticles, often a catalyst used to promote a desired conversion, is ina moving bed that allows the catalyst to be intermittently orcontinuously withdrawn (e.g., for regeneration by burning accumulatedcoke) and replaced in order to maintain a desired level of catalyticactivity in the reactor. Therefore, the screens may, for example,confine the catalyst in the radial direction (e.g., between an innerradius and an outer radius of an annular retention zone formed by theupstream and downstream screens) but still allow the catalyst to moveaxially in the downward direction. It is often desired in processesaccording to the present invention to maintain an axially downward flowof the solid particles, normally from a particle (e.g., fresh orregenerated catalyst) inlet at the top of the particle retention zone toa particle (e.g., spent catalyst) outlet at the bottom of this annularzone. Processes incorporating radial flow, moving bed catalytic reactorsare therefore representative of those which may benefit from a reductionin fluidization according to aspects of the invention as describedherein. Catalyst addition and withdrawn may be performed continuously orintermittently.

The terms “upstream” and “downstream” refer to the positions of thescreens relative to the direction of horizontal (e.g., radial) fluidflow, such that the upstream screen is first contacted with this fluid,followed by the downstream screen. After contacting the upstream screenand before contacting the downstream screen, this horizontally flowingfluid contacts the solid particles disposed between these screens. Theterms “axial” and “radial” are used to designate directions with respectto cylindrical vessels, such that, when these vessels are orientedvertically, the “axial” direction is a vertical direction parallel tothe axis of the cylinder, whereas the “radial” direction is a horizontaldirection along a radius of a circular, horizontal cross section of thecylinder. It is understood that the terms “axial” and “radial” can morebroadly refer to vertical and horizontal directions in cases of methodsas described herein being performed in vessels and/or with screenshaving shapes other than cylindrical shapes.

FIG. 1 illustrates an offset between the vertical (e.g., axial)positions of upstream openings 16 a of upstream screen 10 a anddownstream openings 16 b of downstream screen 10 b. As shown, upstreamscreen 10 a has upstream openings 16 a that extend above (e.g., are at agreater axial height relative to) highest downstream openings 16 b′ ofdownstream screen 10 b. This offset tends to impart a downward flowcomponent to fluid (e.g., a hydrocarbon feed or oxygen-containingcatalyst regenerant) after entering the bed of solid particles or solidparticle retention zone 14 through upstream screen 10 a at upstreamopenings positioned higher than the highest downstream openings.Therefore, while an upstream flow direction 18 of all, or at least aportion (e.g., greater than about 50%, and often greater than about90%), of the fluid that contacts solid particle retention zone 14 issubstantially horizontal (e.g., substantially radial) prior to enteringupstream screen 10 a, this fluid exhibits a downwardly biased flow 12,having a downward (e.g., axial) flow component at least in an upperregion of the solid particles above highest downstream openings 16 b′ ofdownstream screen 10 b but below highest upstream openings 16 a′ ofupstream screen 10 a. The downward bias in fluid flow hindersfluidization of solid particles near the top of solid particle retentionzone 14, thereby mitigating some or all of the drawbacks, as discussedabove, that are associated with fluidization in radial flow fluid/solidcontacting processes. For example, an effective reduction influidization can be achieved when highest upstream openings 16 a′ areabove (i.e., are at an axial height exceeding) highest downstreamopenings 16 b′ by a vertical height differential (e.g., an axialdistance) generally from about 0.5 times to about 2 times, and typicallyfrom about 0.8 times to about 1.8 times, a horizontal spacing distancebetween the screens, corresponding to the width of the bed of solidparticles or solid particle retention zone 14.

Advantageously, reduced fluidization allows for a reduction in theamount of a sealing portion of the solid particles, if used inconjunction with the processes described herein. In the cross-sectionalview of the embodiment illustrated in FIG. 1, a base of sealing portion14 a of solid particle retention zone 14 is defined by a line segmentconnecting highest upstream openings 16 a′ to highest downstreamopenings 16 b′, which base also corresponds to the arrow in FIG. 1showing downwardly biased flow 12. Sealing portion 14 a of solidparticles therefore has a diagonal base and extends to the top of bothupstream and downstream screens 10 a, 10 b and consequently to the topof solid particle retention zone 14. Sealing portion 14 a also has a topfree surface 25, all or a portion of which is generally above highestupstream openings 16 a′ of upstream screen 10 a. By offsetting upstreamand downstream screens 10 a, 10 b as described herein, a minimum depthof sealing portion 14 a, i.e., the vertical, for example axial, distancefrom highest upstream openings 16 a′ of upstream screen 10 a to the topof solid particle retention zone 14, is generally less than thehorizontal spacing distance between the screens, or width of the solidparticle retention zone 14. Typically, this minimum depth of sealingcatalyst is less than about 0.8 times, and often less than about 0.5times, this width.

Offsetting of the screens alleviates the problem, as discussed above, ofupward fluid flow near the top of solid particle retention zone 14,and/or in sealing portion 14 a of this zone, which would otherwiserender the solid particles susceptible to fluidization. It has beendetermined that, according to particular embodiments of the invention,none of the solid particles become fluidized if at least some, asparticles defining top free surface 25, reside above highest upstreamopenings 16 a′ of upstream screen 10 a. Preferably all of top freesurface 25 resides above highest upstream openings 16 a′, as shown inFIG. 1. These findings, associated with offsetting the upstream anddownstream screens, are exploited in processes of the present invention,not only in terms of easing concerns related fluidization, but also ofallowing a reduction in depth of the sealing portion 14 a of the solidparticles. A reduction in bed depth can translate to an increase in unitcapacity in the case of a revamp of an existing radial flow process, orotherwise a reduction in overall elevation of the solid particleretention zone 14 and associated equipment (e.g., a vessel and screens).

Alone or in combination with sealing portion 14 a of the solidparticles, a sealing portion of the fluid (e.g., a hydrocarbon feed oran oxygen-containing catalyst regenerant) may be introduced fromdirectly above the solid particle retention zone 14 and passed throughthe zone, often having an annular configuration, with a substantiallyaxially downward flow 22. Unlike the sealing portion of the fluid,therefore, the major portion of the fluid has a substantially horizontal(e.g., radial) upstream flow direction 18. The flow of the sealingportion relative to the flow of total fluid introduced into the bed ofsolid particles (in any flow direction) (e.g., the flow of the sealingportion relative to the combined flow of the sealing portion and themajor portion flowing substantially horizontally) is generally less thanabout 10%, typically less than about 5%, and often less than about 2% ofthis total or combined flow. The sealing portion of the fluid generallyhinders fluidization by exerting a downward force on particles at ornear the top of the particle retention zone 14. In some cases, however,fluidization may be adequately controlled even without the use of asealing portion.

As noted above, a significant consideration is the ability to impart adownward flow component in fluid flowing through the bed of solidparticles at or near axial elevations corresponding to highest upstreamand downstream openings 16 a′, 16 b′ of upstream and downstream screens10 a, 10 b. Importantly, the advantages in terms of reduced fluidizationcan be realized without the need to induce a specific pressure dropprofile (e.g., a graduated pressure drop as a function of axial length)over the entire, or even a significant, vertical height of upstreamand/or downstream screens 10 a, 10 b. Therefore, the pressure dropacross at least one of the upstream and downstream screens is minimized,and in preferred embodiments is less than about 10% (e.g., in the rangefrom about 0.01% to about 10%) of the pressure drop across the bed ofsolid particles. Typically, this pressure drop is less than about 5%,and often less than about 3%, of the pressure drop across the bed ofsolid particles. Otherwise the pressure drop from (i) the point that themajor portion of fluid, flowing substantially horizontally, firstimpacts any part connected to the upstream and/or downstream screen(e.g., to induce a pressure drop) or other type of particle retentionelement to (ii) the point that this major portion of fluid first entersthe solid particle retention zone 14, or first contacts the solidparticles may also be within these percentage ranges, relative to thepressure drop across the bed of solid particles.

Preferred radial flow processes according to the present inventioninvolve the use of cylindrical, vertically oriented vessels that containthe upstream and downstream screens that are also cylindrical anddisposed concentrically. In such embodiments, solid particles areretained between the screens in an annular particle retention zone. Theupstream and downstream screens have the ability to not only retainsolid particles such as catalysts but also effectively distributeradially or horizontally flowing fluids to these particles. Although thescreens are described herein primarily with respect to their exemplaryuse in radial flow reactors, it is understood that the advantages ofoffsetting the screens is broadly applicable to a wide variety ofapparatuses and methods for fluid/solid contacting. Illustrativeexamples include filtration, selective gas or liquid adsorption (e.g.,pressure swing adsorption or the adsorptive separation of liquids),reactive distillation, and others.

A representative screen, which may be used as an upstream screen or adownstream screen, is depicted in FIG. 2. Screen 10 has a cylindricalshape that may be positioned, for example concentrically, within aradial flow reactor having an outer, cylindrical vessel (not shown). Aplurality of openings 16 in this embodiment are flow channels formed onan outer surface as gaps between a plurality of elongated particleretention elements 4. Retention elements 4 are spaced apart along theaxial length of the cylindrical screen, which may correspond to theaxial length of the vessel in which the screen 10 is disposed. As shownin FIG. 2, openings 16 in the form of flow channels may have constantwidths at the outer surface of the screen and be spaced apart atconstant intervals. Both the elongated particle retention elements 4 andflow channels 2 formed between them extend circumferentially in circularshapes at constant axial positions and spaced apart in the axialdirection, with these shapes corresponding to the circular cross sectionof the cylinder. It is possible for the circumferentially elongatedparticle retention elements 4 to extend about the perimeter, or at leastpart of the perimeter, of other shapes, for example ovals or polygons,defining a radial boundary of an adjacent particle retention zone 20.Fluid from the interior or exterior of screen 10 is therefore directedin a radial or horizontal flow direction through openings 16 in the formof flow channels, passing from the outer surface to an inner surface.According to further embodiments, the elongated particle retentionelements and flow channels formed between them may extend axially ratherthan circumferentially.

A representative type of circumferentially elongated particle retentionelement 4 used for upstream and/or downstream screen 10 is profile wire.Profile wire screen is often fabricated with the wires surrounding, andwelded to, a cage of longitudinal support rods 6. The profile wire, as atype of circumferentially extending particle retention element 4,extends about the perimeter (circumference) of a circle defining aninner or outer radial boundary of annular particle retention zone 20disposed between screen 10 and another upstream or downstream screen(not shown). Typically, the profile wire has a triangular cross section,with triangular bases forming a smooth surface on one side of theprofile wire screen, which is normally the side adjacent to the solidparticles, and triangular vertices forming an edged surface on theopposite side of the screen, which is normally the side adjacent an openspace such as an annular or central fluid distribution zone or fluidcollection zone, as described below with respect to hydrocarbonconversion processes.

In an alternative embodiment, a single elongated particle retentionelement 4 such as profile wire is wound in a spiral shape, and openings16, namely flow channels, are formed between adjacent, spaced apartwindings. In this embodiment, flow channels are effectively formedbetween each complete turn of the windings to provide the equivalent ofmultiple flow channels, although actually only a single flow channelextends in the spiral shape. While the flow channels formed in thisembodiment extend circumferentially, they are not horizontal as in theembodiment of FIG. 2, but are instead somewhat pitched in the axialdirection. The axial pitch may deviate from a constant axial position(e.g., constant horizontal height or plane), for example by less thanabout 25°, and often less than about 10°.

Still further embodiments of the invention are directed to hydrocarbonconversion processes utilizing radial flow reactors that contain theupstream and downstream screens having an offset configuration and thepressure drop characteristics as discussed above. In exemplaryprocesses, a major portion of a hydrocarbon feed stream, at leastpartially in the gas phase, is passed, either substantially radiallyinward or substantially radially outward, through an annular bed ofsolid catalyst particles disposed between the screens. Outflow operationis preferred. A minor sealing portion of the hydrocarbon feed may bepassed axially downward from the top of a sealing portion of thecatalyst (i.e., the “seal catalyst”) through the annular catalyst bed.The processes further comprise withdrawing a hydrocarbon productresulting from the contact between both the major portion and sealportion of the hydrocarbon feed with the hydrocarbon conversioncatalyst. Exemplary processes also comprise adding regeneratedhydrocarbon conversion catalyst to the sealing portion of the catalystand withdrawing spent hydrocarbon conversion catalyst from the bottom ofthe annular catalyst bed. The adding and withdrawing of the catalyst maybe performed, for example, either continuously or intermittently.

The hydrocarbon product stream may be withdrawn, for example, from thecatalyst bed through a cylindrical fluid collection zone at the centerof the reactor (e.g., internal to downstream screen, in the case of aninwardly flowing hydrocarbon feed stream, or otherwise through anannular fluid collection zone at an inner periphery of the reactor(e.g., external to the downstream screen), in the case of an outwardlyflowing hydrocarbon feed stream. A representative radial flow process isused to dehydrogenate paraffinic hydrocarbon streams containing asaturated hydrocarbon, or a combination of saturated hydrocarbons, inthe C₃ to C₂₂ carbon number range, to provide a correspondingmono-olefin or combination of mono-olefins. Another representativeprocess is a catalytic reforming process used to increase the octanenumber of a naphtha petroleum fraction, or a hydrocarbon feed streamcomprising hydrocarbons boiling in the range from about 80° C. (180° F.)to about 205° C. (400° F.).

Further representative embodiments of the invention are directed toradial flow reactors, including moving bed reactors, comprising a vesseland screens having a offset configuration, as described herein. Thescreens are disposed in the vessel to promote the desired fluid/solidparticle contacting. In many cases, the vessel and screens will becylindrical, with the vessel and screens being positionedconcentrically, and often with their common axes extending vertically.Other geometries for the vessel and/or screens, for example, conical, orcylindrical with one or more conical ends, are possible. The screens mayalso be used in reactors having cross-sectional shapes that are notcircular, for example oval or polygonal. Normally, the cross-sectionalshapes of the vessel and screens will be the same (although smaller insize in the case of the particle retention device) at any common axialposition within the vessel, in order to promote radial flow uniformity.

As discussed above, however, the use of both outer and inner screens canbe advantageous for not only distributing the inlet fluid such as ahydrocarbon-containing feed to, but also for collecting the outlet fluidsuch as a hydrocarbon-containing product from, the particle retentionzone 20. FIG. 3 illustrates the use of both upstream screen anddownstream screen 10 a, 10 b, both disposed in a radial flow reactor 100and having an offset configuration with respect to their openings, asshown in FIG. 1. Cylindrical vessel 5 and screens 10 a, 10 b are alloriented vertically as shown and disposed concentrically with respect toeach other. Particle retention zone 20 in this case is an annular zonebetween upstream and downstream screens 10 a, 10 b. The annular spaceexternal to downstream screen 10 b and within cylindrical vessel 5 may,as discussed above, be a fluid collection zone 22 a for recovering fluidexiting the particle retention zone 20 through downstream screen 10 b inthe case of fluid flowing outwardly as shown by arrows in FIG. 3, or,alternatively, may be a fluid distribution zone in the case of fluidflowing radially in the opposite direction (i.e., inwardly). Conversely,the central space, in the fluid flow configuration shown in FIG. 3 maybe a central fluid distribution zone 22 b for distributing fluid axiallyto upstream screen 10 a or may be a central fluid collection zone in thecase of radial fluid flow in the opposite direction. Flow arrows in FIG.3 illustrate radial fluid flow through upstream and downstream screens10 a, 10 b and also through annular particle retention zone 20, but anoverall downward flow of feed distributed to, and product collectedfrom, the particle retention zone 20.

Overall, aspects of the invention are associated with offset screenssuch as profile wire screens, in which an upstream screen has openingsabove (i.e., at a greater axial height relative to) highest downstreamopenings in a downstream screen for use in radial flow reactors.Representative profile wire screens have a smooth first surface definedby triangular bases and an opposing second surface defined by triangularvertices extending radially and spaced apart axially. Those having skillin the art, with the knowledge gained from the present disclosure, willrecognize that various changes could be made in the above screens, aswell as radial flow fluid/solid particle contacting apparatuses andprocesses utilizing these screens, without departing from the scope ofthe present disclosure.

The following example is set forth as representative of the presentinvention. This example is not to be construed as limiting the scope ofthe invention as other equivalent embodiments will be apparent in viewof the present disclosure and appended claims.

EXAMPLE 1

Fluidization of solid particles was studied in radial flow fluid/solidcontacting. A vertically oriented cylindrical vessel containingvertically oriented, cylindrical upstream and downstream screens andsolid particles retained in an annular space between the screens wasused in the study. Radially flowing fluid was passed through theupstream screen, particle bed, and downstream screen in experiments inwhich the screens (i) had an offset configuration, with openings in theupstream screen being above the openings in the downstream screen and(ii) did not have this configuration, with highest openings in bothscreens being at approximately the same axial height.

The results showed that, using the offset screen configuration, theaverage depth of a sealing portion of the catalyst could be reducedrelative to the alternative of using the non-offset or conventionalconfiguration. In fact, using the offset screen configuration, even anominal amount of solid particles above the highest upstream openingsthe upstream screen was sufficient to prevent particle fluidization.

1. A process for converting a fluid oxygen-containing catalystregenerant at least partially in the gas phase, the process comprising:(a) passing a major portion of the oxygen-containing catalyst regenerantsubstantially radially through solid particles of hydrocarbon conversioncatalyst retained in an annular catalyst bed between concentric,cylindrical upstream and downstream screens; (b) passing a minor sealingportion of the oxygen-containing catalyst regenerant axially downwardfrom the top of a sealing portion of the catalyst and through theannular catalyst bed; (c) withdrawing a spent regenerant resulting fromcontact between the major portion and sealing portion of theoxygen-containing catalyst regenerant with the hydrocarbon conversioncatalyst; (d) adding spent hydrocarbon conversion catalyst to thesealing portion of the catalyst; and (e) withdrawing regeneratedhydrocarbon conversion catalyst from the bottom of the annular catalystbed; wherein the upstream screen has openings above highest downstreamopenings of the downstream screen, wherein a base of a sealing portionof the solid particles is defined by a downwardly sloping line segmentfrom highest upstream openings to the highest downstream openings,wherein a ratio of a) a vertical distance from highest upstream screenopenings to a top of the sealing portion to b) a horizontal spacingdistance between the upstream screen and the downstream screen is lessthan about 0.8, and wherein a pressure drop of the fluid across each ofthe upstream and downstream screens is less than about 10% of a pressuredrop across the bed of the solid particles.
 2. The process of claim 1,wherein the pressure drop of the fluid across the upstream screen isless than about 5% of the pressure drop across the bed of the solidparticles.
 3. The process of claim 1, wherein highest upstream openingsare above the highest downstream openings by a vertical heightdifferential from about 0.5 to about 2 times the horizontal spacingdistance between the screens that defines a depth of the bed of thesolid particles.
 4. The process of claim 3, wherein the vertical heightdifferential is from about 0.8 to about 1.8 times the horizontal spacingdistance between the screens.
 5. The process of claim 1, comprisingpassing the fluid radially within, and with respect to, a verticallyoriented vessel containing the upstream and downstream screens.
 6. Theprocess of claim 5, wherein the vertically oriented vessel and theupstream and downstream screens are cylindrical.
 7. The process of claim6, wherein the upstream and downstream screens have flow channels formedfrom one or more elongated particle retention elements.
 8. The processof claim 7, wherein the elongated particle retention elements extendcircumferentially about the axes of the cylindrical upstream anddownstream screens.
 9. The process of claim 8, wherein the elongatedparticle retention elements comprise profile wires having a triangularcross section, and wherein the flow channels are formed between adjacentwires.
 10. The process of claim 5, comprising passing the fluid radiallyinward from a peripheral fluid distribution zone, external to theupstream screen, and to a central collection zone, internal to thedownstream screen.
 11. The process of claim 5, comprising passing thefluid radially outward from a central fluid distribution zone, internalto the upstream screen, to a peripheral fluid collection zone, externalto the downstream screen.
 12. The process of claim 6, wherein the solidparticles are retained in an annular particle retention zone between theupstream and downstream screens, the process further comprising flowingthe solid particles axially.
 13. The process of claim 12, the processfurther comprising passing a sealing portion of the fluid axiallydownward through the annular particle retention zone.
 14. The process ofclaim 1, wherein the ratio a) to b) is less than about 0.5.